One example of a power transmission system comprising a plurality of transmission units having different responsiveness to a speed change is disclosed in Japanese Patent Laid-Open No. 2003-127681. According to the hybrid drive system disclosed in Japanese Patent Laid-Open No. 2003-127681, an internal combustion engine is connected with a carrier of a planetary gear mechanism, and a first motor/generator is connected with a sun gear of the planetary gear mechanism. Also, a ring gear is connected with a member of an input side of a geared automatic transmission. A member of an output side of the automatic transmission is connected with a propeller shaft, and a second motor/generator is connected with the propeller shaft. Thus, according to this drive unit for a hybrid vehicle, the planetary gear mechanism functions as a distribution mechanism distributing an engine power to the first motor/generator and to the transmission.
According to the system disclosed in Japanese Patent Laid-Open No. 2003-127681, a revolution frequency of the engine is varied continuously by changing a revolution frequency of the first motor/generator. That is, the planetary gear mechanism and the first motor/generator function as a continuously variable transmission. Consequently, a total speed change ratio of the drive unit for the hybrid vehicle is determined by the speed change ratio of the planetary gear mechanism functioning as a continuously variable transmission and the gear stage of the transmission arranged on the output side of the planetary gear mechanism.
As explained above, according to the system taught by Japanese Patent Laid-Open No. 2003-127681, the revolution frequency of the engine is varied by changing a revolution frequency of the first motor/generator. Therefore, a substantial speed change ratio of the planetary gear mechanism or the distribution mechanism is varied promptly. However, on the other hand, a shifting operation of the geared transmission is carried out by engaging and releasing frictional engagement devices. This means that the required time for the shifting operation of the geared transmission is relatively longer.
According to Japanese Patent Laid-Open No. 2003-127681, the transmission is provided for the purpose of improving a power transmission efficiency and a fuel economy by reducing a power transmission with a conversion of the power onto an electric power as much as possible even in case the vehicle is running under low load at high speed or running under high load at low speed, and downsizing the motor/generator. For this purpose, the gear stage of the transmission is shifted sequentially to the stage of smaller ratio with an increase in the vehicle speed, as a typical automatic geared transmission. On the other hand, a speed change ratio of the distribution mechanism is varied continuously by the first motor/generator. As a result, a total speed change ratio of the vehicle is set according to a running condition of the vehicle.
Thus, an entire speed change ratio, that is, the total speed change ratio is set by a speed change ratio of the continuously variable transmission unit and a gear stage of the geared transmission unit. However, a speed change operation is carried out in response to a change in the vehicle speed, an output demand, an accelerating operation, a decelerating operation and so on. That is, a content of a speed change demand is different depending on the situation. Therefore, if a speed change operation of the continuously variable transmission and a shifting operation of the geared transmission are always carried out on a consistent basis to approximate the total speed change ratio to the target speed change ratio, a speed change operation may not be carried out in conformity with such various demands concerning the speed change operation.